Field of the Invention
The present invention relates to a method of fabricating a light extraction substrate for an organic light-emitting device, and more particularly, to a method of fabricating a light extraction substrate for an organic light-emitting device which can increase the extraction efficiency of light emitted from the organic light-emitting device, thereby improving the overall luminous efficiency of the organic light-emitting device.
Description of Related Art
In general, light-emitting devices can be generally divided into organic light-emitting devices in which a light-emitting layer is made of an organic matter and inorganic light-emitting devices in which a light-emitting layer is made of an inorganic matter. An organic-light-emitting device is a self-light emitting device which generates light using energy emitted from excitons that are generated through the recombination of electrons injected through a cathode and holes injected through an anode. Such organic light-emitting devices have a variety of advantages, such as, low-voltage driving, self-light emission, a wide viewing angle, a high resolution, natural color reproduction and rapid response.
Recently, active studies are underway in order to apply organic light-emitting devices to a variety of devices, such as portable information devices, cameras, watches, office equipment, information display windows of vehicles, televisions (TVs), displays, or illumination systems.
Approaches for improving the luminous efficiency of organic light-emitting devices include an approach of improving the luminous efficiency of a material that constitutes a light-emitting layer and an approach of improving the light extraction efficiency at which light generated from the light-emitting layer is extracted.
The light extraction efficiency depends on the refractive indices of the layers which form an organic light-emitting device. In a typical organic light-emitting device, when a ray of light generated from the light-emitting layer is emitted at an angle greater than a critical angle, the ray of light is totally reflected at the interface between a higher-refractivity layer which could be a transparent electrode layer and a lower-refractivity layer which could be a substrate. This consequently lowers the light extraction efficiency, thereby lowering the overall luminous efficiency of the organic light-emitting device, which is problematic.
More specifically, only about 20% of light generated from an organic light-emitting device is emitted to the outside and about 80% of the light is lost by a waveguide effect originating from the difference in the refractive index between a glass substrate and an organic light-emitting diode portion which includes an anode, a hole injection layer, a hole transport layer, an emissive layer, an electron transport layer and an electron injection layer, as well as by the total internal reflection originating from the difference in the refractive index between the glass substrate and the air. Here, the refractive index of the internal organic light-emitting layer ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO) which is generally used for the anode is about 1.9. Since the two layers have a very small thickness ranging from 200 to 400 nm and the refractive index of the glass used for the glass substrate is about 1.5, a planar waveguide is thereby caused inside the organic light-emitting device. It is calculated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate is about 1.5 and the refractive index of the ambient air is 1.0, when the light is directed outward from the inside of the glass substrate, a ray of the light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate. The ratio of the trapped light is about 35%, so only about 20% of the generated light is emitted to the outside.
In order to overcome this problem, in the related art, the surface of the glass substrate is roughened by hydrofluoric acid etching or sandblasting, and then a high refractive index frit is applied on the surface of the glass substrate. This can consequently disturb the internal waveguide mode, thereby improving the light extraction efficiency of the organic light-emitting device. The surface roughness of the glass substrate is required to be great since paths along which light is emitted can be diversified in proportion to the surface roughness. Then, a significant improvement in the light extraction efficiency can be expected.
However, this approach has a limit to increasing the surface roughness of the glass substrate. That is, it was proved that the conventional method does not have a significant effect in improving the light extraction efficiency of the organic light-emitting device.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.